This invention relates generally to a control circuit and associated method of driving a thermal print head of a thermal printing apparatus, and more particularly to a control circuit providing a control pulse sequence to activate the print head and improve the rate of heat input thereto.
Thermal printers utilize thermal print heads to transfer print data to a thermally sensitive media. A print head has an active surface containing resistive elements, the elements are activated or xe2x80x9cheatedxe2x80x9d by applying a control voltage to the resistive elements of the print head. A control circuit activates the print head with a control voltage pulse of sufficient duration to cause the resistive elements of the print head to heat to a desired temperature. Upon activation, the elements are brought into contact with the thermally sensitive media, typically a recording media or a toning transfer ribbon. In this way, thermal energy is transferred from the print head by conduction to the ribbon or recording media.
As a thermal print head is activated, much of the heat produced by the resistive elements is retained in the print head resulting in a significant temperature rise of the print head, as many elements are typically activated in a short period of time. Repeated activation of the print head by the control circuit results in residual heat energy contributing to the overall thermal energy transferred to the ribbon or recording media. Therefore, less additional energy is required from a control circuit to produce an impression on a recording media with a such xe2x80x9cwarmxe2x80x9d print head. As such, many modem print heads incorporate thermistors or other devices that provide a measurement of the temperature of the print head. The energy to the warmed resistive elements can then be proportionately reduced by reducing the length of time the warm resistive elements are activated. This is done in a manner that provides for relatively constant energy per impression to the ribbon or print media.
Some thermal printers also use control circuit logic to determine how much energy to supply to a resistive element and then change the length of time the resistive element is activated accordingly. This is done by adding up the activations of resistive elements over given lengths of time, converting the time to energy delivered, and calculating the temperature rise of the print head. The conversion from time to energy delivered is possible because the thermal properties of the print head and its surrounding area are known. The local temperature rise in the area of resistive elements that will subsequently be used can also be calculated enabling the use of individualized voltage pulse widths to make impressions at the proper thermal energy levels.
The operation of thermal print heads has also been advanced by providing preheat current to the resistive elements. i.e., providing a small amount of current to the resistive elements to bring the temperature of the resistive elements up to a level that is just below the operating temperature required to make an impression on the recording media. This allows a minimum amount of additional energy to activate the print head to make an impression on the media, and maximizes the speed of the printer. The additional amount of energy required to reach the operating temperature depends on the degree of prior usage and the resultant temperature of the print head. Such parameters are either determined with a temperature measuring sensor of the control circuit or by counting prior resistive element activations and calculating the current print head temperature.
Presently, the efficiency in thermal print head operation has been limited to the aforementioned methods of improving the amount of energy necessary to reactivate a previously active or warm head. It is desirable for a control circuit to improve the rate of heat input to a thermal printing head such that the heating time of the thermal print head is reduced independent of the temperature of the print head prior to activation.
The present invention is directed to a method of activating a print head to further increase the speed of a thermal printer by maximizing the rate of heat input into a resistive element independent of the temperature of the print head prior to activation.
Briefly stated, the present invention provides a control circuit for driving a thermal print head of a thermal printing apparatus. The control circuit improves operation of a thermal printing apparatus by maximizing the rate of heat input into a thermal print head, thereby improving the printing cycle time independent of the temperature of the print head prior to activation. The control circuit applies a first high voltage pulse to the print head, then switches to a second voltage pulse lower in potential than that of the first pulse during the energizing period to prevent the element from being driven to a damaging temperature.
Specifically, the control circuit includes a switch which is operably linked to a power source and to the print head of the thermal printing apparatus to provide a control pulse sequence to the print head. The control pulse sequence includes a first pulse and a second pulse. The first pulse has a first electrical potential and is applied to the print head for a first duration to heat the print head to the desired temperature for activating the print mode. The second pulse has a second electrical potential lower than the electrical potential of the first pulse and is of a second duration for maintaining the printing temperature of the printing head. A control circuit timer is provided for operating the switch at the end of the first pulse to provide the second pulse. In this way, the pulse control sequence provides an improved rate of heat input to the print head by decreasing the time and energy required for the print head to attain the predetermined printing temperature and maintaining this temperature for the duration of the second pulse.
Additionally, a method of driving a print head of a thermal printing apparatus is provided wherein a first electrical potential of a control pulse sequence is applied to the print head for a first duration, then the electrical potential applied to the print head is switched from the first electrical potential to a second electrical potential. The second electrical potential is lower than that of the first electrical potential and is applied to the print head for a longer duration to provide an improved rate of heat input to the print head.